The porous bottom culture dishes (PBCDs) and related devices developed in this laboratory for the sterile measurement of electrophysiological parameters of cell layers are used in nearly 100 laboratories. Two companies are making commercial versions of the PBCDs using cellulose ester membranes (Millipore Corp.) and polycarbonate membranes with and without cell culture treatment (Costar Corp.) Most are used for epithelia cell layers. Recently, we have grown endothelial and smooth muscle cell layers on our PBCDs made with our transparent collagen membranes. These show promise of being good models of several types of blood vessels. The major role played by Ca in the regulation of many cellular processes has prompted us to improve methods for measuring free Ca activity in cell systems. We have developed a 2 mm diameter electrode using a hydrophobic porous membrane and neutral carrier Ca exchanger which has a response time of 10 seconds and a resistance of 10 megohms. It is useful for measuring small quantities of solutions representative of cell interiors for many biochemical studies.